Many layers of thin films are used to make an integrated circuit. IC manufacturing requires thin films to have certain properties in order for the circuit to function as designed. For example, there is a general need for materials with low dielectric constants (low-k). Using low-k materials as the intermetal dielectric (i.e., the layer of insulator separating consecutive levels of the conductive metal interconnects) reduces the delay in signal propagation due to capacitive effects, otherwise know as the RC delay. A dielectric material of low dielectric constant will have low capacitance, and hence the RC delay of an IC constructed with such a material will be lower as well.
As another example, there is a general need for materials with specific tensile or compressive stresses. Increasing shallow trench isolation (STI) film tensile stress increases transistor drain current and device performance because the electron and hole mobilities are higher. Other applications require dielectric films to have compressive stress. These and other properties may be met on the film as deposited, or after treatment.
One such treatment may be a thermal process in which the substrate is heated to a temperature for a time. A thermal treatment may remove unwanted particles from the film, or change its stresses and other properties. These thermal processes, however, have certain difficulties. In particular, substrate temperatures generally need to be high (i.e., greater than about 500 degrees Celsius) with exposure times typically on the order of hours. As is well known in the field, these conditions can damage copper containing devices, especially in the application wherein a low-k dielectric is being cured and the long exposure time may be unsuitable for mass manufacturing. Also, the use of temperature sensitive nickel silicide precludes inducing film stress by using temperatures above 400° C. while some SiN films have a cure temperature up to 480° C.
To overcome these disadvantages of thermal processing, another technique has been developed, which involves curing the film with UV radiation. Irradiation of the low-k or spacer nitride films permits modulation of desired film properties such as dielectric constant or film stress at lower temperatures. However, the use of UV radiation in such processes may result in deleterious side-effects unless special care is taken to deliver the precise amount of radiation and to maintain the substrate temperature at an optimal level. Delivery of the precise amount of radiation may be complicated when the deposited film on the substrate is not perfectly uniform. A uniform UV cure may exacerbate pre-existing non-uniformity by curing less the areas that need more film property change and curing more the areas that need less film property change.
What are needed therefore are improved methods and devices for treating thin films with UV radiation to obtain desired film properties.